(1) Field of the Invention
The present invention relates to an image forming apparatus and a developing device for developing an electrostatic latent image on an image carrier using a developer, the image forming apparatus including the developing device.
(2) Description of the Related Art
An image forming apparatus, such as a copier or a printer, includes a developing device which contains a two-component developer including carriers and toner, and develops an electrostatic latent image formed on a photosensitive drum.
Examples of a developing method include a so-called trickle developing method. In this method, a developer including fresh carriers (i.e., fresh developer) is supplied into a housing of a developing device little by little via an inlet of the housing. At the same time, developer in the housing is agitated and circularly conveyed. While the developer is being conveyed, an excess amount of developer due to the supply of the fresh developer overflows and is discharged outside the housing via an outlet of the housing (Japanese examined patent application publication No. H02-21591).
According to the trickle developing method, the fresh developer is supplied into the housing from a cartridge, and whereby old carriers in the housing are replaced by fresh carriers and discharged outside the housing. Therefore, deteriorated carriers are less likely to remain in the housing, and deterioration in the developer is suppressed for a long period of time. This realizes high image quality in forming images.
Although realizing high image quality, the use of a developing device for the trickle developing method as described above is costly. This is because such a developing device requires a large amount of fresh developer for supply, and a two-component developer is discharged from the developing device.
Suppose here that a user sometimes needs to print out images having high image quality, such as pictures, but other times needs to print out images that do not require high image quality, such as images including only characters. In this case, the user may not wish to use the developing device for the trickle developing method all the time, since it is costly although providing high image quality.
Here, a normal developing method, which is not the trickle developing method, is less costly. In this method, a two-component developer in a housing is not discharged via an outlet, and a fresh developer including toner and not carriers is supplied into the housing from a cartridge. However, such a normal developing method cannot provide high image quality for a long period of time.
For example, it is possible to prepare two developing devices, i.e., one for the trickle developing method and the other for a normal non-trickle developing method. Then, the user may switch between the two developing devices as necessary. This structure, however, is costly as it requires two types of developing devices. Also, the user needs to keep one of the developing devices not mounted in the image forming apparatus somewhere different from the image forming apparatus, which is bothersome.